I have a 5 year old North Face Aleutian 20°. I absolutely love it, but only use it for car camping since it's heavy compared to my down bags. I stored it in the included stuff sack for about two years, then loose in a big pillow case the last 3 years. I just got it out and seems to be like new. I guess I would have to test it sometime to be sure.
This topic reminds me of those like "iPhone vs Andriod" and "Ponchos vs rain jacket/pants" - seems like there will never be a true answer. I sure wish someone had been testing two identical bags stuffed vs. hanging for the past 10 years and gave them a thorough test...just to give some clarity.